A communication apparatus generally comprises a signal transmit chain and a signal receive chain connected to an antenna and to a baseband processing device.
FIG. 1 shows a communication apparatus 1 according to the prior art comprising a transmit chain Tx and a receive chain Rx connected to an antenna ANT and a baseband processing unit 2, notably a baseband processor.
The transmit chain Tx notably comprises a power amplifier device 3 (“PA”), an output of which is connected to the antenna ANT by the intermediary of a transistor T1. The common node between the amplifier device 3 and the transistor T1 is connected to ground GND by the intermediary of a transistor T2. The input of the amplifier device 3 is connected to other conventional and known elements of the transmit chain (not shown here in order to simplify the figure).
The receive chain Rx comprises an amplifier device 4 commonly denoted by those skilled in the art by the name “low noise amplifier” (“LNA”) an input of which is connected to the antenna ANT by the intermediary of a transistor T3. The common node between the antenna ANT and the amplifier device 4 is connected to ground GND by the intermediary of a transistor T4. The output of the amplifier device 4 is connected to other conventional and known elements of the receive chain (not shown in order to simplify the figure).
The amplifier device conventionally comprises at least one amplification transistor T.
The transistors T1, T2, T3 and T4 can be of the same type, for example thick gate oxide transistors, and can be of different sizes. They are of any type whatsoever, for example NMOS field effect transistors. The switching circuit is controlled by a controller incorporated in the baseband processing unit 2.
In signal transmit mode, the transistors T1 and T4 are on. They behave like parasitic resistances Ron. The transistor T4 provides the grounding of the input of the amplifier device 4. The transistors T2 and T3 are off. They behave like parasitic capacitances Coff.
The transistor T4 forms, in transmit mode, an element for discharging the switching circuit of the receive chain.
In receive mode, the transistors T1 and T4 are off and the transistors T2 and T3 are on. The transistor T2 provides the grounding of the output of the amplifier device 3. The transistors T1 and T4 behave like parasitic capacitances Coff. The transistors T2 and T3 behave like parasitic resistances Ron. These parasitic elements degrade the noise figure of the amplifier device 4.
The quality of the device is described by the coefficient Coff*Ron. However, this coefficient is fixed for a given technology.
In transmit mode, the amplifier device 3 transmits for example a signal of power 33 dBm, that is to say 2 W, at a voltage of 10V. The parasitic capacitance of the transistor T3 allows a parasitic current to flow, which supplies the amplifier device 4.
The current resulting from the parasitic capacitance of the transistor T3 damages the amplification transistor T if the protection given by T4 is not sufficient. Consequently the switching transistors are sized such that they channel the parasitic power well below the voltage of 10V. They therefore have appropriate dimensions.
Generally, each switching transistor T1, T2, T3 and T4 comprises a set of transistors assembled in series and in parallel in the form of a matrix. This arrangement makes it possible to find functional and non-destructive configurations for a given Ron*Coff and limits the voltage liable to damage the amplifier device 4.
However, it is desirable to find a compromise between the overall dimensions of the matrix and the improvement of the protection of the amplification transistor T.
The different elements of the switching circuit are moreover separated from each other by a few micrometers. Consequently, electromagnetic and substrate coupling phenomena appear. These phenomena also degrade the low noise figure of the amplifier device.